This invention relates to a treatment facility of the type that includes a granular media bed to remove suspended solids or other impurities from a liquid mixture, and that employs a backwash operation between treatment operations to restore or otherwise enhance the quality of the bed.
It is common practice to clean treatment facilities, such as filters including a granular media bed, by employing a backwashing operation between successive filter runs, that is, between successive filtering operations in which the impurity containing liquid, such as industrial and municipal water or wastewater, is directed through the granular bed to remove suspended solids. The backwashing action in such a filter essentially reverses the filtration process, and removes the impurities from the surfaces and interstices of the granular material forming the filter bed. Specifically, the liquid employed in the backwash operation, ordinarily water from a clear well or other suitable source, is forced through the filter bed opposite to the direction of filtration to both lift and agitate granules of the bed. This action releases impurities from the bed that were retained during the filter run. The cleaning effect during backwashing is greatly enhanced by causing the media granules to collide on a frequent basis, and this is best achieved by just completely fluidizing the entire portion of bed to be cleaned. It is in this fluidized condition that the individual media granules are, at the same time, closely enough spaced to each other so that a high probability exists for frequent collisions, and far enough apart to release the entrapped impurities.
Obtaining a high degree of granule collisions in the backwashing of the filter is extremely desirable; particularly when suspended solid impurities, in addition to being trapped in the media bed interstices, actually cling to the individual granules. The scrubbing and scouring action between granules resulting from their collision with each other is an effective mechanism for removing the latter referred to impurities.
In U.S. Pat. No. 2,858,024, issued to Russell H. Babcock on Oct. 28, 1958, a control system is provided to sense the essential cessation of the change in differential pressure through a sand filter bed as the backwash rate is being increased. The backwash rate at which this cessation takes place represents the minimum backwash rate at which the media bed is fully fluidized and is the rate for maximizing collisions between the sand granules. However, once this rate is reached Babcock suggests that it should be maintained for the entire backwashing operation. Although this may be the most desirable rate for separating impurities from the media itself, in most cases it is lower than the desired rate for removing the separated impurities from the filter chamber or basin. Therefore, carrying out the backwash operation as suggested by Babcock can result in unnecessarily prolonging the backwash operation, and thereby unduly increasing the "down" time of the filter (i.e. the time that the filter is unavailable for filtering). Moreover, after backwashing in the manner suggested by Babcock, an undesirably high quantity of impurities may remain in the filter because the backwash rate, while being effective to separate the impurities from the media, may not be effective to totally transport the impurities out of the filter chamber.
It has been recognized in the prior art that the viscosity of the backwash liquid plays an important role in the backwashing operation. Specifically, it is recognized that the backwash rate or velocity will need to be changed in response to changes in backwash liquid viscosity in order to obtain a desired level of bed expansion, as well as to transport impurities out of the filter chamber. It is also known that the viscosity of a backwash liquid, such as water, will vary with temperature; the viscosity of warm water being lower than that of cold water. Accordingly, to obtain the same desired degree of bed expansion it is necessary to employ a greater backwash rate of flow with warm water than with cold water.
Both U.S. Pat. Nos. 1,892,951 (Hughes) and 2,376,912 (Green) recognize that the viscosity of water is temperature dependent, and that the effect of different viscosities should be taken into account in backwashing a filter. Both Hughes and Green suggest that the best method of controlling the backwash operation is to vary the backwash rate, as is necessary, to achieve a constant, or desired level of filter bed expansion. To accomplish this result both patentees employ mechanical devices to detect when the bed has been expanded to a predetermined level. Applicants have found that such mechanical devices are not very reliable, and introduce undesirable mechanical components into the filter system.
Although many different backwash control systems have been devised, none of them, to the best of applicants' knowledge, have optimized both the separating function (i.e. the function of separating impurities from the interstices and surfaces of the granular media) and the removal function (i.e. the removal from the filter of the separated impurities) in a two stage backwash operation of the type forming the subject matter of the present invention.